First of all, thanks for your answer.
Yes, I set the mode to be a mixed Authentification, it is possible at least by modifying a registry key.
I 'm now able to create a user but i can not manage to open its default db.
here are my commands :
sp_grantdbaccess 'loginname', 'loginname'
but when i try to connect, i have the following error :
Can not open default database. Login failed.
I test it on a real SQL server before doing my tests on MSDE.
When I look to it, my db does not have my new user assigned.